Measuring the SUV (Standardized Uptake Value) for PET scans

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Dear OncoLink "Ask The Experts," I have had three PET scans for my malignant melanoma. My first one was April 2000 to evaluate me for Interferon; it was negative. The second was October 2001 because I had another lymph node swollen. That PET was negative except for that node. My last one June 2002 showed low activity in legs and neck. What is the activity Measuring the SUV (Standardized Uptake Value) level of concern? I heard that the activity is graded 0-20.

Peeyush Bhargava, MD, Chief Fellow in the Department of Nuclear Medicine at the University of Pennsylvania, responds:

PET scans are complex studies involving a lot of variables related to the patient and to the technique. The images show the distribution and metabolism of FDG (fluoro-deoxy glucose) throughout the body. Cancers show up as foci/areas of intense activity since cancer cells are more active and take up a lot of FDG.

There can be other causes of intense activity such as inflammation, infection, or muscle activity. Measuring the SUV (Standardized Uptake Value) is a semi?quantitative way to assess the activity in a given focus. It represents the amount of activity in the focus relative to the activity in the other (normal) areas of the body. Typically, the SUV value of metastatic lesion is above 2.5. The pattern of activity (the distribution of lesions in the different parts of the body) is also very important as different cancers have different preferential sites of metastasis. Also, patient's history and findings on other anatomic imaging like CT scans and MRI are taken into consideration while reading the PET scans.
Below are the images from a patient's PET scan (coronal images, anterior to posterior), which show intense activity in the different muscle groups. This patient did not fast long enough before the study was performed. This caused increased FDG uptake in the muscles and can lead to an incorrectly interpreted study.